Keywords: buried pipelines, regression, horizontal bend, vertical bend, buckling, temperature gradient.

Analysis and design of buried pipelines are usually based on classical theories and semi-empirical methods. One of such methods is the API recommended practice 1102 [1] to design pipelines which are to be located under highways and railroad crossings. The confidence in these approaches is very limited and use of them could lead to a very conservative design, which could cause major increases in costs. The major objective of this research is to develop software which can handle the major limitations that are present in the classical procedures.

Complete, comprehensive and lengthy numerical analyses using the finite element methods were carried out to calculate stresses and strains of buried pipes. Many parameters were considered in this investigation; they included soil weight, live load, impact factor, paved and unpaved roads, trench angle, construction increment, soil type and density, variable truck load, and temperature effect on pipelines without vertical or horizontal bends. Additional parameters included pipe diameter, D/t ratio, cover height, bend radius, bend angle, internal pressure, fluid specific weight, and temperature gradient.

In order to be able to handle the large amount of generated data from the different finite element runs in the proposed software, regression equations were developed to predict the absolute maximum stress and diameter change of buried pipes. Regression equations were developed for pipes crossing both paved and non-paved highways which are subjected to variable truck loads, pipes with horizontal and vertical bends, and pipes which are subjected to variable sand overburden heights. The equations developedcovered three types of soils, namely: sand, marl, and sabkha. Linear multiple regression was used to build the relationships between the independent and the dependent variables. The independent variables were the pipe's diameter, thickness of the pipe's wall, and depth of cover. The dependent variables were the absolute maximum stress and the change in the pipe's diameter. Prior to the development of the regression equations, a number of data points were selected randomly. These data were not used in developing the models, but were used in the generated models to test the accuracy of the models in predicting the dependent variables correctly [2]. In addition, comprehensive statistical analysis tests were used to evaluate the adequacy of the generated models.

The personal computer software, using Visual Basic 6.0, was developed utilizing the results obtained. The results of the regression analysis were used to develop three modules, which contain subroutines and functions to perform the following tasks: Design and Analysis of Straight Pipes which are subjected to sand overburden or crossing highways that are subjected to live loads, Design and Analysis of Horizontal or Vertical Bends, and Pipe Buckling Checks.

1

API Recommended Practice 1102, "Steel Pipelines Crossing Railroads and Highways", American Petroleum Institute, Washington, D.C., 1993.

2

D.C. Montgomery, E.A. Peck, "Introduction to Linear Regression Analysis", John Wiley & Sons, 1982.

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